Entrainer Selection for Homogeneous Azeotropic Distillation

Separation of a mixture of isopropyl alcohol () and water is impossible with a single distillation column because of the existence of a minimum-boiling homogeneous azeotrope (, ). This Demonstration illustrates how the addition of an extraneous component, called an entrainer, can facilitate the separation. The entrainer alters the relative volatilities of the components and thereby "breaks" the azeotrope. Homogeneous azeotropic distillation generally uses a heavy entrainer, which does not introduce new azeotropes.

In this Demonstration, we evaluate two entrainers: dimethyl sulfoxide () and ethylene glycol (). The nonideal thermodynamic properties of the ternary system are accounted for using a Wilson model (parameter values are taken from the Aspen thermodynamic database). The pseudo-equilibrium curve [1] represents the mole fraction of on a entrainer-free basis in the vapor phase () versus its liquid phase counterpart (). In this plot, the blue and red curves are the pseudo-equilibrium plots for with the addition of and , respectively. As the amount of entrainer is increased, the location of the intersection of the pseudo-equilibrium plot with the 45° line (represented by a cyan () or orange () dot) (see snapshot 3) moves to higher mole fractions of , and then eventually disappears. By varying the amount of entrainer, it is clear that the cyan dot disappears first, which indicates that is the preferred entrainer (i.e., lower entrainer to azeotropic feed molar flow ratio, and lower energy consumption in the entrainer recovery column), resulting in a more economical separation.

This optimal entrainer is confirmed by looking at equivolatility diagrams [2, 3, 4]. The position of the green and magenta dots on the - edge of the triangular diagram indicates that less is required to break the azeotrope. Additional information, consistent with the results obtained by plotting the pseudo-equilibrium data, can be obtained form the equivolatility diagram: (1) for both entrainers, will be the distillate product of the extractive column, since the isovolatility curve intersects the - edge; (2) higher relative volatilities are obtained with , indicating that it is a better entrainer than ; and (3) the abscissa of the magenta (green) dot ( and for and , respectively) corresponds to the minimum amount of () needed to break the azeotrope. Though the equivolatility diagram is quite sensitive to the thermodynamic model chosen, the choice of which is a better entrainer is not.